Logarithm Basics Part III

Algebra Level 5

log 2 ( 2 x 2 ) × log 4 ( 16 x ) = log 4 x 3 \large \color{#D61F06}{\sqrt{\log_2(2x^2) \times \log_4(16x)}=\log_4x^3}

If a 1 , a 2 , . . . a n a_1,a_2,...a_n are all the real roots of the above equation, then find the value of a 1 + a 2 + . . . + a n n \dfrac{a_1+a_2+...+a_n}{n}


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The answer is 16.000.

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3 solutions

Parth Lohomi
May 12, 2015

The given equation can be expressed as

( 1 + 2 log 2 x ) ( 2 + log 2 x 2 ) = 3 2 log 2 x \sqrt{(1+2\log_2x)(2+\frac{\log_2x}{2})}=\frac{3}{2}\log_2x

Let log 2 x = m \log_2x=m ,then

( 1 + 2 m ) ( 2 + m 2 ) = 3 2 m \sqrt{(1+2m)(2+\frac{m} {2})}=\frac{3}{2}m

( 1 + 2 m ) ( 2 + m 2 ) = 9 4 m 2 (1+2m)(2+\frac{m} {2})=\frac{9}{4}m^2

2 + 9 2 m = 5 4 m 2 2+\frac{9}{2}m=\frac{5}{4}m^2

5 m 2 18 m 8 = 0 5m^2-18m-8=0

( m 4 ) ( 5 m + 2 ) = 0 (m-4)(5m+2)=0

m = 4 , 2 / 5 m=4,-2/5 \implies x = 16 , 2 2 / 5 x=16,2^{-2/5}

We reject 2 2 / 5 2^{-2/5} \therefore x = 16 x=\boxed{16}

Thus the answer is 16 1 = 16 \dfrac{16}{1}=\boxed{16}

\square

Why did we reject 2 2 / 5 2^{-2/5} ?

Abhishek Sharma - 6 years, 1 month ago

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Because it will make the R.H.S. a negative, which is impossible in case of real numbers.

Sandeep Bhardwaj - 6 years, 1 month ago

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But isn't 2 5 -\frac{2}{5} in the range of log 2 \log_{2} ?

Jake Lai - 6 years, 1 month ago

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@Jake Lai What do you mean?

Pi Han Goh - 6 years, 1 month ago

Thank you.

Abhishek Sharma - 6 years, 1 month ago

I dont think we should reject as square root a quadratic equation will get you positive and negative roots...

Zack Yeung - 6 years, 1 month ago
Noel Lo
May 28, 2015

Similar method as Parth Lohomi but then I initially accepted both roots. Then I just guessed that the non-integral root should be rejected then presto! Anyway thank you Sandeep Bhardwaj for enlightening me on why that root should be rejected!! :)

Can someone tell me why did we have to ignore the (0.5)^0.4 part? Sandeep Bhardwaj sir

Root function always has positive range so the rhs in the origonal equation must be a positive number.

Satvik Choudhary - 5 years, 11 months ago

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